Cellular communication systems are capable of providing not only voice services, but also mobile broadband services. As the number of applications supported by cell phones continues to increase resulting in greater amounts of data consumption, the need for mobile broadband data services also increases. This requires telecommunication operators to improve data throughput wherever possible.
As the spectrum efficiency for the point-to-point link already approaches its theoretical limit, one way to increase data throughput is to split big cells into smaller and smaller cells. When cells become closer to each other, however, adjacent cell interferences become more severe, and the cell splitting gain saturates. Furthermore, today it is becoming increasingly difficult to acquire new sites to install base stations for the operators and the associated costs are also increasing. Therefore, cell-splitting cannot fulfil all of the demands necessary to provide the demand for increased broadband services.
Recently a new type of network deployment, so-called HetNet (Heterogenous Network), has been proposed and is attracting a lot of interest and effort in the industry. In HetNet, another tier consisting of multiple Low-Power Nodes (LPN's) is added onto an existing macro base station's coverage. In some examples of this deployment, the macro base station works as a master and the Low Power Nodes (LPN) work as slaves in order to have better interference management and resource allocation, etc. In other deployments, however, the LPN's work as equals with the macro base station, i.e. the LPN's are not subservient to the macro base station.
One deployment choice for example in a Long Term Evolution (LTE) network, is a deployment in which the underlying Low Power Nodes do not have their own cell ID (identification). Instead, the Low Power Nodes all share the same cell ID with the Macro station in this deployment.
In LTE networks, the position of the cell specific reference signal (CRS) is deduced from the cell ID. The CRS is transmitted from the Macro node. The LPN's may or may not transmit the CRS. These and other conventional deployments suffer from several disadvantages or problems as discussed in further detail below.
In general, the LTE network sends control channels to the user equipment (UE) communicating via the network. The control channel is sent from a central point. A control channel carries no user data. Instead, the control channel is used to configure the communication with the UE. The UE then decodes a number of control channels. A UE determines if a decoded control channel is dedicated to it, if a cyclic redundancy check (CRC), or similar code, matches one of a set of UE identity numbers such as an RNTI (Radio Network Temporary Identifier) or other identifier.
This control channel mechanism can be used for many purposes, including delivering uplink scheduling grants to a UE.
These control channels can be either Unicast or Broadcast. In Unicast control channels, the UE identity number is unique. A UE may have multiple unique UE identity numbers. The use of a particular identity number could be used to indicate a certain type of scheduling, for example dynamic or semi-persistent scheduling (SPS). In Broadcast control channels, all UE's in a cell share a UE identity number, i.e., all the UE's in a cell have the same UE identity number. This is done to deliver system information to all UE's. In LTE networks, for example, all UE's within a cell share the same system information radio network temporary identifier (SI-RNTI).
One problem that can arise during an LTE uplink transmission is described below. However, the same or similar problem also exists for LTE downlink transmissions, or even uplink or downlink transmissions in accordance with other networks and other communication standards.
With the introduction of HetNets, the available radio resources can be reused several times by sharing them among the LPN's. FIG. 1 shows an example of a cell that includes macro base station 1 and LPN's 5. The illustrated example is an example in which macro base station 1 and LPN's 5 share the same cell ID. In the illustrated example, three LPN's 5 are deployed under the same macro cell 7 and in the illustrated example, each LPN 5 is associated with a user equipment, UE 9. Each LPN 5, i.e. LPN1, LPN2 and LPN3, has an associated LPN uplink coverage region 17 and the associated LPN uplink coverage regions 17 are separated from each other within macro cell 7. One UE 9 is illustrated in each of the LPN coverage regions 17 in FIG. 1 but multiple UE's are typically associated with each LPN coverage region 17. Uplink PUSCH (physical uplink shared control channel) transmissions 13 of the UE's 9 can be carried out with low power since they only need to reach the closest LPN 5 and not macro base station 1. As such, the uplink PUSCH transmissions 13 of an LPN coverage region 17 therefore do not interfere with other UE transmissions such as other UE transmissions from other UE's 9 to other LPN's 5 in other LPN coverage regions 17. Hence, the uplink transmissions can be carried out on the same physical resources and more UE's can be served simultaneously when compared to only the macro cell covering that area. By the “same physical resources,” it is meant that the same physical resource blocks (PRB's) can be used by several UE's; for uplink transmission. If the UE's are located so that their uplinks are not interfering with each other, then the UE's can transmit using the same PRB's. This technique is oftentimes referred to as space division multiple access (SDMA) or area splitting.
When more UE's 9 are served simultaneously, more control signaling is needed to schedule the transmissions of these UE's. For example, the uplink scheduling information in LTE systems is transmitted to the UE's with Downlink Control Information (DCI). In LTE, the DCI signals 19 are sent either on the physical downlink control channel (PDCCH) (from 3GPP Release 8 and onwards, for example) or on the enhanced PDCCH (ePDCCH) (from 3GPP release 11, for example). The available resources for PDCCH/ePDCCH resources are limited and have not been extended in the recent 3GPP releases. This becomes a major bottleneck when multiple UE's need to be served simultaneously. The scheduler, which is typically implemented in the macro cell 7, decides which UE's 9 are scheduled on the physical resources and which modulation coding scheme (MCS) each UE 9 has to use. This information is then sent as a downlink control information (DCI) signal 19 to the corresponding UE 9. The DCI signals 19 includes a UL grant, e.g. UL grant 1 (DCI), as indicated in FIG. 1. The UL grant identifies which PRB's (physical resource blocks) should be used by the associated UE for uplink transmission.
One example of a problem that can arise in networks such as networks described above, is as follows.
A DCI such as DCI signal 19 consists of one or several control channel elements (CCE's). Depending on the length of the DCI and the radio link quality, 1, 2, 4 or 8 CCE's are used to form one DCI in some examples. The number of total available CCE's is limited and depending on the system bandwidth and the number of OFDM symbols that are used for PDCCH in the sub-frame. In many examples, there are 1-3 OFDM symbols per sub-frame. OFDM (orthogonal frequency-division multiplexing) is a method of encoding digital data on multiple carrier frequencies and OFDM symbols can represent or convey one or several bits of data.
In some examples, such as when a system's bandwidth is 10 MHz, 9, 26 or 42 CCE's may be available depending on if 1, 2 or 3 OFDM symbols are used for the PDCCH space. The number of OFDM symbols may be limited for the PDCCH since they take away resources for data transmission. For example, if 4 CCE's are needed to form one DCI, and only 26 CCE's are available, then only 6 DCI's can be sent during one sub-frame. Additionally, there are separate DCI's for power control, DL (downlink) and UL (uplink) scheduling. The control channel mechanism delivers uplink (UL) scheduling grants to a UE via DCI signal 19. Thus, under these conditions, only 2-3 UE's can be scheduled for uplink transmission during one sub-frame and it would be desirable to schedule more UE's for uplink transmission.
The present disclosure addresses such shortcomings.